The functions of the approximately 98% of the human genome that do not encode human cellular proteins remain to be elucidated. Actively replicating endogenous retroviruses entered the human genome millions of years ago and became a stable part of the inherited genetic material, with retroviral elements presently making up approximately 8% of the modern human genome. These viruses subsequently acquired multiple mutations, leading to the widely-held assumption that they are no longer competent to replicate. However, in studying living patients rather than the standard cell lines, we have recently discovered surprising evidence suggesting that in certain patients with cancer HERV-K (HML-2), an endogenous retrovirus that is a relatively recent entrant into the human genome and has been linked to oncogenesis, might still be capable of replication. Replication and transmission of endogenous retroviruses is difficult to prove using standard techniques, however, as these viruses are already present in the genomes of all human cells. Therefore, we have assembled a diverse and expert group of investigators who will test the hypothesis that modern HERV-K (HML-2) can replicate using cutting-edge and complementary techniques. We will use a newly devised molecular system in which antibiotic resistance serves as a surrogate marker to assess whether we can passage virus from the blood of patients in the laboratory. We will also use high-throughput DNA sequencing and Bioinformatics to find full-length functional virus that is present in cancer patients but is not represented in the current draft of the human genome. An infectious clone that is representative of modern HERV-K (HML-2) will be made by using the information gained from the Bioinformatics studies and/or by cloning a full-length virus from the HERV-K RNA found in the blood of patients. This clone will then be tagged with green fluorescent protein and used to directly study replication. Building on our advances using Nuclear Magnetic Resonance (NMR) to visualize spatially correlated dynamics that direct RNA and protein conformational transitions, we will also develop NMR methods to visualize HERV-K (HML-2) replication in vivo. Proof that endogenous retroviruses can still replicate in modern humans will lead to a paradigm shift in thinking about these viruses, and will suggest a role for them in reshaping individual genomes. In addition, as increased expression of chromosomal endogenous retroviral sequences has been linked to cancer and autoimmunity, these findings will be relevant to understanding the pathogenesis of significant diseases. Finally, if replicating, infectious endogenous human retroviruses capable of causing disease can be found in the blood of given individuals, this information will have important implications for the safety of the blood supply.